Ample evidence exists for transcriptional modification of gene expression in response to hypoxia. Prokaryotes and yeast switch to an aerobic metabolism to conserve vital components of cell growth while mammalian cells, unable to survive under anaerobic conditions, attempt to attenuate this common stress. The long term objective of this proposal is to understand the molecular mechanisms involved in the transcriptional response to hypoxia. Mammalian cells are exposed to hypoxia under conditions that vary for each cell type. Some cells are chronically hypoxic due to their distance from blood high in oxygen content, e.g. deep medullary cells of the kidney and centrolobular cells of the liver. Other cells are well oxygenated at rest, but under conditions of heavy use are subjected to hypoxic stress: skeletal and cardiac muscle. Tissues of the renal cortex and of the central nervous system are protected from hypoxia by their proximity to and perfusion with blood high in oxygen content. Hypoxia as a pathologic stress underlies many types of injury and disease. Disruption of blood supply, a decrease in oxygen carrying capacity, and/or failure to adequately saturate blood with oxygen produce hypoxia or anoxia. Hypoxic injury occurs in myocardial infarction, cerebrovascular disease, pulmonary disease, acute tubular necrosis, infection, and trauma. Hypoxia is common in neoplasia as a consequence of disordered tissue and blood vessel architecture. The sequelae of hypoxia depend on the cell type, and the duration, extent and severity of hypoxia. Erythropoietin (Epo) is a human gene that is tightly regulated at a transcriptional level by hypoxia. The regulation involves, in part, cis-acting elements in the Epo promoter and enhancer that conform to consensus steroid hormone receptor response elements. The mechanism by which the steroid response elements contribute to the hypoxic response will be evaluated by studying DNA sequence requirements for binding and function of transactivating factors, the effect of expression of steroid hormone receptors on Epo gene activity, and the requirement for steroid-like ligands. The specific transactivating factor(s) that bind(s) and activate(s) the Epo gene will be isolated by purification and/or molecular cloning. Uncovering the molecular events that regulate cellular responses to hypoxia will broaden our understanding of a universal physiologic and pathologic stress, and may permit design of molecular therapeutics that are specific for hypoxia.